The present invention generally relates to apparatus and methods for treatment of airstreams in an Environmental Control System (ECS) to adjust concentrations of air constituents. The constituents include natural air compounds as well as compounds that may cause odor.
ECSs of various types and complexity are used in military and civil airplane, helicopter, and spacecraft applications. In aircraft for example, airflow from outside the aircraft supplied through engine or APU bleed systems or other air sources including ground supplies and electric compressors, is circulated to occupied compartments, cargo compartments, and electronic equipment bays. The air delivered to cabin and other occupied compartment is usually a mixture of similar amounts of outside air and recirculated cabin air. The quality of this air is generally very good, especially at cruise altitudes and during normal operating conditions.
The source of the outside air is compressed engine air. The cost of this outside air is related to the fuel that the engine uses to compress the air without the benefit of generating thrust. Typically, the compression of outside air in the engine and its conditioning by the ECS is up to 3% of the total fuel burn for the flight. In the 1970's, NASA's studies concluded that recirculating cabin air could reduce fuel burn without compromising air quality. Since then, the ratio of recirculated air into the cabin is roughly 50%, but still provides a high flow rate of outside air per passenger. As the recirculation ratio increases above 50%, there is a possibility that the concentration of compounds that cause odors (volatile organic compounds, or VOC's) or carbon dioxide (CO2) from passenger exhalation, may impact the air quality. Another aspect of interest is the cabin pressure, which is determined in part by the pressure and flow rate of compressed engine air. Aluminum aircraft typically have a cabin pressure equivalent to an 8,000 foot altitude, while that of newer, composite aircraft are at 6,000 feet. The cost of pressurizing the cabin increases with lower equivalent altitudes, not only due to the cost of outside air, but to the strength and weight of the fuselage required to contain the increased pressure. Finally, outside air may contain VOC's, especially during ground operations, that may cause smell-in-cabin (SIC) incidents, which may lead to unscheduled and unwarranted maintenance, flight disruptions, and passenger discomfort.
Therefore, it would be desirable to increase the recirculation rate while maintaining or even improving the current air quality.